Japanese Patent No. 3578311 and Japanese Laid-Open Patent Publication No. 2005-140503 each disclose a vehicle weight detecting apparatus that detects the total weight of a vehicle or the weight of a load carried by the vehicle.
According to the weight detecting apparatus of Japanese Patent No. 3578311, an air pressure sensor for detecting the internal air pressure of a tire, a temperature sensor for detecting the internal temperature of the tire, and a transmitter, which wirelessly transmits signals representing the detected internal air pressure and internal temperature, are provided on each of vehicle wheels. A receiver, which receives signals from the transmitters, and a measuring apparatus, which computes the weight of the load carried by the vehicle based on the signals received by the receiver, are provided on the vehicle body. The measuring apparatus previously stores a table data, which sets the relationship between the internal air pressure of the tire and load applied to the tire. The measuring apparatus obtains the internal air pressure and the internal temperature of each tire from the signals received by the receiver, and corrects the value of the detected internal air pressure based on the internal temperature to obtain a corrected air pressure. Then, the measuring apparatus computes the total weight of the vehicle and the weight of the load carried by the vehicle based on the corrected air pressure using the table data.
According to the weight detecting apparatus of Japanese Laid-Open Patent Publication No. 2005-140503, a load sensor is provided between the rim of each vehicle wheel and the tire mounted on the rim. The load sensors each generate a voltage of a level corresponding to the load (tire load) applied to the tire from the rim. Also, a transmitter unit is mounted on each wheel. Each transmitter unit includes an air pressure sensor, which detects the internal air pressure of the tire, and a temperature sensor, which detects the internal temperature of the tire. Each load sensor is connected to the associated transmitter unit. Each transmitter unit wirelessly transmits signals representing the internal air pressure and the internal temperature detected by the air pressure sensor and the temperature sensor, and wirelessly transmits signals representing a value of voltage generated by the associated load sensor. A receiver unit is provided on the vehicle body. The receiver unit receives signals from the transmitters and computes the weight of the load carried by the vehicle based on the received signals. The receiver unit previously stores a map data, which sets the relationship between the output voltage value of the load sensor and the tire load for different internal air pressures of the tire. The receiver unit obtains the internal air pressure and the internal temperature of each tire from the received signals, and computes corrected air pressure by correcting the value of the internal air pressure based on the internal temperature. The receiver unit also obtains the output voltage value of each load sensor from the received signals, and computes the tire load using the map data based on the output voltage value and the corrected air pressure. Then, the receiver unit computes the total weight of the vehicle based on the tire load.
In the apparatus of Japanese Patent No. 3578311, the total weight of the vehicle and the weight of the load carried by the vehicle are computed using the table data, which sets the relationship between the internal air pressure of each tire and the load applied to each tire, based on the detected internal air pressure of the tire. That is, in the apparatus of Japanese Patent No. 3578311, based on the assumption that the internal air pressure of the tire changes in accordance with changes in the load applied to the tire, the total weight of the vehicle and the weight of the load carried by the vehicle are computed based on the internal air pressure of the tire. However, since the internal air pressure of the tires changes only slightly with respect to changes in the load applied to the tires, it is realistically difficult to accurately compute the total weight of the vehicle and the weight of the load carried by the vehicle from the relationship between the internal air pressure of the tire and the load applied to the tire. Thus, the apparatus of Japanese Patent No. 3578311 is not practical.
In the apparatus of Japanese Laid-Open Patent Publication No. 2005-140503, it is necessary to provide load sensors to detect the tire load. Thus, the configuration of the apparatus is complicated and the costs are increased. In addition, in the case in which the load sensor is provided between the rim of each wheel and the associated tire, air is more likely to leak from between the rim and the tire. Moreover, the operation for mounting the tires on the wheels becomes very cumbersome. Furthermore, in particular, when the vehicle is travelling, since sudden and great dynamic load is applied to the load sensors, it is not preferable to use the load sensors under such harsh conditions in the aspect of durability.